1. Field of the Invention
The present invention relates to compositions for preventing flow capacity damage to wellbore screens and slotted liners. More specifically, the present invention relates to a coating that is applied to wellbore screens or slotted liners prior to insertion of the screens or slotted liners into the wellbore. The coating is comprised of reactive materials mixed with a binder. The coating protects the screens and slotted liners during insertion into the wellbore.
Also, after the screens and slotted liners have been placed in the well, the coating dissolves or melts off of the screens or slotted liners and the reactive materials within the coating react with the wellbore fluids and remaining drill-in fluid filtercake to degrade them, thus preventing the screens and slotted liners from becoming plugged with the fluids and filtercake and thereby allowing the well to achieve optimum production. The term “drill-in” is used herein to define the fluid used to drill into the producing interval of the well and may differ from the composition of the drilling fluid used in creating the rest of the wellbore.
2. Description of the Related Art
Several types of screens and liners are used in poorly consolidated formations to allow production of oil or gas. Fluids pass from the formation through the screens and up the tubing and out of the well. Unconsolidated formation materials bridge on the screen material or external gravel pack and are prevented from passing into the tubing.
The production of the well is highly dependent on having the flow paths in the screen or liner remain open, thus allowing the well to produce at its designed rate. However, often the screen will become plugged with bridging agents, polymers, and drill solids that are left in the wellbore at the completion of the drilling operation. The screen or liner may become plugged during placement of the screen as it is being pushed through remaining drilling mud or may become plugged once it is in place when the formation and remaining filtercake collapse or are carried with the produced fluids or gas onto the screen. This is particularly a problem in high angle or horizontal wellbores where it is very difficult to successfully circulate cleaning treatments such as acids or enzymes around the screen to remove remaining drilling fluid filtercake prior to beginning production.
As a result, when production begins, the remaining drilling fluid filtercake may be carried into the screen along with the reservoir liquids or gas and can plug or restrict production from large areas of the screen.
Various blends of waxes and mixtures of wax and polymers have been used in the past to coat the screen or liner in order to protect the flow channels of the screen or liner while the screen or liner is being placed into the wellbore. See U.S. Pat. Nos. 3,880,233 and 3,905,423 for examples of wax coatings that might be suitable for such use. These waxes generally were designed to melt at the wellbore temperature to restore the flow capacity of the screen. Also, some of these coating materials were also soluble in crude oil. Other temporary coating materials have been used for this purpose, including meltable or dissolvable fiber-reinforced wax, an active metal which may be dissolved by acid or caustic solution, salts, asphaltenes that are dissolvable in crude oil, and eutectic compounds formed by combining organic compounds. U.S. Pat. No. 5,320,178 teaches a fiber-reinforced plug. U.S. Pat. No. 5,310,000 teaches an active metal wrapped base pipe. U.S. Pat. No. 5,165,476 provides examples of compounds that might be used as temporary coating materials. U.S. Pat. No. 4,202,411 provides an example of an acid soluble coating.
Until now, none of these coating materials have been designed to also include materials which can be released after the screen or slotted liner is in place and react with or solubilize plugging materials that may be in the wellbore.
The present invention addresses this need in that it uses a coating binder, such as solid surfactants, waxes, blends and the like, in which to incorporate reactive materials, such as enzymes, chelants, organic acids, surface active agents, oxidizers and other reactive materials which are capable of dissolving, degrading, or dispersing potentially plugging materials, such as for example materials commonly found in drilling fluids, filtercakes formed from these drilling fluids, hydraulic fracturing fluids, and high viscosity completion fluid “pills”.
One object of this invention is to provide a coating on the screen or liner that serves to prevent damage to the screen or liner while the screen or liner is being inserted into the wellbore.
A further object of this invention is to release the coating from the screen or liner at a controlled rate.
Another object of this invention is that the coatings can be customized for the particular well environment and can also be designed so that the coating is released from the screens or liners sequentially. For example, the coating may be varied along the length of a well so that the coating on the distal end of the string of screen is released from the screen prior to release of the coating on the proximal end of the string of screen.
Still a further object of this invention is to release reactive materials from the coating that will react with and dissolve or disperse materials that are commonly found in drilling fluids or in the filtercakes formed from these fluids.
Another object of the present invention is to release reactive materials into the wellbore that serve to degrade high viscosity completion fluid “pills” which have been placed into the wellbore to control fluid loss during a completion.
Still another object of the present invention is to degrade fluids used to place external gravel packs around the screen, i.e. gravel pack carrying fluids.
A further object of the present invention is to release reactive materials into the wellbore that serve to degrade hydraulic fracturing fluids that have been used in a “frac and pack” application where slurries of fracturing fluid and particles have been pumped into the formation and pumped between the screen and the wellbore casing to pack the annulus.
Another benefit of this invention is that it may eliminate the need for a wash pipe to be placed inside the screen in order to pump fluids down to the end of the screen and circulate the wellbore. If a wash pipe is not employed, the binding agent for the coating will be selected in such a way as to remain in place until the pumping process is completed.